We have discovered that a simple set of small nuclear RNA species is covalently linked to chromosomal DNA in HeLa cells (Pederson and Bhorjee, J. Mol. Biol. 128, 451-480, 1979). Our present studies are aimed at determining precisely how and where these RNA's are integrated into the chromosomal DNA, as mapped by psoralen cross-linking and electron microscopy. We are especially interested in the possibility that these RNA's play a role in the tertiary structure of chromatin in the cell nucleus.